1. Field of the Invention
The invention relates to a method for operating a communication system for communication between a central station like a service center and one or a plurality of remote objects like vehicles, trucks, airplanes, and plants, houses and/or other such facilities that include a battery-based electric power supply. The invention further relates to a computer program for conducting this method, as well as for affecting the objectives of the disclosed invention(s).
2. Background of the Invention
Remote objects are usually equipped with an independent power supply that comprises (includes) a battery and an electric generator for supplying on board electric and electronic devices and other electrical equipment with electric energy. The generator is driven by a motor when the object is operated. In periods when the object is out of operation, however, many devices nevertheless have to be active or at least in a sleep mode. For such periods, the battery is provided for supplying these devices with the necessary electric energy. From time to time the battery has to be re-charged by means of the electric generator according to the amount of energy consumed therefrom.
A common problem with such a battery power supply resides in the risk that a discharge of the battery may impair the operation, readiness and/or safety of the object, if the object as well as the electric generator are not operated for a longer time. For avoiding this risk, it is known to monitor the battery output-power and to give an alarm if the battery is going to be discharged so that appropriate actions for re-charging the battery (e.g. starting the motor so that the electric generator can re-charge the battery) can be taken. However, in case of remote objects with no personnel or staff, or in the case of a vehicle that is parked in a remote area for a longer time, it is often difficult or expensive to handle such a situation. This problem is especially serious if a communication system on board the object is used, for example, for monitoring, controlling or locking/unlocking the object and it functions via a central station and which has to be in an active mode permanently for communicating with the central station.
U.S. Pat. Nos. 5,251,325 and 5,392,457 disclose a battery saving method and apparatus for selectively supplying power to each receiver function of a battery powered communication receiver at predetermined time intervals for enabling the reception of coded message signals with each receiver function being powered only as long as is required for each receiver function to stabilize prior to the reception of said coded message signals.
EP 0319219 discloses a paging apparatus with a battery saving function wherein a power supply control means is provided for periodically switching on a receiving means for a predetermined period in response to the detecting of synchronization signals and in the absence of a predetermined signal sequence, and for switching off the same when the predetermined signal sequence is detected and immediately after reception of a paging signal for the apparatus.
However, in case of a communication system comprising a central station like a service center and a plurality of remote objects, a random switching of such objects between on- and off-status, or between wake and sleep modes or periods can cause problems with respect to availability of the objects, as well as with respect to the network load if at one time too many objects are simultaneously in a sleep or wake period.